The effects of the thymosin polypeptides on T-cell differentiation pathways will be studied using murine bone marrow and thymus cells from young and aging animals. Studies will be performed on purified subpopulations of bone marrow and thymus cells isolated by a combination of lectin agglutination and density gradient techniques. In vitro studies will involve short term incubation with purified thymosin peptides alpha 1, alpha 5, alpha 7, beta 3 and beta 4 of normal bone marrow cell subpopulations, and subpopulations of thymocytes from normal or hydrocortisone acetate (HCA) treated animals. Criteria for induction of T-cell differentiation and/or maturation will include: The expression or disappearance of T-cell associated markers (TdT and 20-alpha SDH enzymes, Thy-1 and Lyt 1.2.3 antigens) and T-cell functional activity (helper cells, suppressor cells, MLR, CML, response to mitogens). Additional parameters of T-cell differentiation will include homing patterns of 51 Cr-labelled cell subpopulations and potential for generation of T-cell clones in soft agar. The use of HCA treated animals will enable a comparison of thymus repopulation by precursor T-cells in young and aging animals. The influences of the purified thymosin pepties on purified subpopulations will be studied using the individual biologically active peptides singly, in concert or sequentially. The effects of the thymosin peptides and antibodies to these peptides, administered in vivo to young and aging animals, will also be studied using the same criteria for T-cell differentiation described above. A radioimmunoassay will be used to monitor changes in the serum thymic hormone levels in normal and treated young and aging animals.